CME CME Motors, Controls & Industrial Wiring

Question 1

Per NEC 430.52, a NEMA Type D motor with a full-load current of 30A is protected by an inverse-time circuit breaker. What is the maximum allowable overcurrent device rating?

Accepted Answer

250% of FLA (75A)

Answer

150% of FLA (45A)

Answer

175% of FLA (52.5A)

Answer

400% of FLA (120A)

Question 2

What is the primary function of a motor overload relay in a motor branch circuit?

Accepted Answer

Protect the motor windings from sustained overcurrent due to overloading or locked rotor

Answer

Provide short-circuit protection for the branch circuit conductors

Answer

Disconnect the motor during ground faults

Answer

Limit inrush current during motor starting

Question 3

Which reduced-voltage starting method is most appropriate for a large centrifugal pump motor where torque requirements are low at startup?

Accepted Answer

Autotransformer starting

Answer

Part-winding starting

Answer

Across-the-line starting

Answer

Synchronous condenser method

Question 4

Per NEC 430.24, conductors supplying several motors must have an ampacity of at least the sum of all motor FLAs plus what additional percentage of the largest motor?

Accepted Answer

25%

Answer

15%

Answer

10%

Answer

20%

Question 5

What does NEMA enclosure type 4X provide that a standard NEMA 4 enclosure does not?

Accepted Answer

Corrosion resistance in addition to watertight and dust-tight protection

Answer

Explosion-proof protection for hazardous locations

Answer

Higher impact resistance for outdoor industrial environments

Answer

Protection against internal condensation

Question 6

When a variable frequency drive (VFD) is used to control a motor, why must the motor nameplate current rating be derated?

Accepted Answer

VFDs produce harmonic currents that cause additional heating in motor windings beyond what sinusoidal supply would cause

Answer

VFDs increase the motor's synchronous speed above nameplate RPM

Answer

VFDs reduce the available torque at all speeds

Answer

VFDs eliminate the need for overload protection, allowing higher continuous current